A typical semiconductor device such as, for example, a MOS type field effect transistor fabricated on a semiconductor substrate is provided with a passivation film to protect the active areas thereof from impurities, moisture and scratches. The passivation film is typically formed of phosphorus-doped silicon dioxide or phosphosilicate glass because the phosphorus-doped silicon dioxide inhibits diffusions of sodium impurities and because it softens and flows at a temperature ranging between 1000 degrees and 1100 degrees in centigrade for creating a smooth topography. However, in another implementation is frequently used a passivation film formed of silicon nitride which is chemically deposited by a plasma-assisted chemical vapor deposition technique at a high frequency, because of its excellent scratch protection. Then, the silicon nitride passivation film is widely used in MOS type field effect transistors.
However, a problem has been encountered in the prior-art MOS type field effect transistor with the silicon nitride passivation film formed by the high-frequency plasma-assisted chemical vapor deposition in deterioration in device characteristics due to hot carriers injected into the gate oxide film from the drain-substrate junction where a strong electric field tends to be applied. Namely, when a passivation film of silicon nitride is formed by the high-frequency plasma-assisted chemical vapor deposition, the resultant passivation film contains a large amount of hydrogen which sometimes ranges between 20 and 30% by atom, because the silicon nitride is formed by reacting silane and ammonia which is assumed to be EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2 (Eq. 1)
The hydrogen adsorbed in the passivation film is diffused into the gate oxide film and, then, reacts to the hot carriers injected thereto for formation of surface states and fixed charges. This results in deterioration in device characteristics such as, for example, the threshold voltage of the MOS type field effect transistor.
Attempts have been made to reduce the amount of hydrogen adsorbed in the passivation film. One of the attempts is to form the passivation film in a high temperature ambient or to apply heat to the passivation film after formation. However, the maximum temperature is restricted below 450 degrees in centigrade because of heat attack to aluminum wiring layers incorporated in the MOS type field effect transistor. Then, the adsorbed hydrogen is slightly reduced but remains on the order of 15% by atom. Other approaches are to use a reaction of silane and nitrogen and a reaction of silicon tetrachloride or silicon tetrafluoride and nitrogen. The silicon nitride film deposited by reacting silane and nitrogen still contains hydrogen of about 15% by atom, and, on the other hand, the silicon nitride film deposited by reacting silicon tetrachloride or silicon tetrafluoride and nitrogen contains a extremely small amount of hydrogen but contains a substantial amount of chlorine or fluorine which tends to corrode metal wiring layers of the MOS type field effect transistor. As a consequence, those attempts could not provide a sufficient solution of the problem inherent in the prior-art MOS type field effect transistor.